Packages and particularly pouches having a peelable seal are well known in the medical field for containing sterile equipment. A peel pouch is an envelope or pouch often times constructed by welding two films together around three sides leaving one end open. An object is then placed inside the pouch, and the pouch then sealed on the fourth side. The pouch usually has an end with generous flaps which may be held in the hands and pulled apart. When this is done, the pouch peels open, revealing its contents in a manner such that they may be removed without any additional contact with the exterior surfaces of the pouch. This peel characteristic is accomplished by using a special adhesive to bond the films together, such that the adhesive strength is lower than the tear strength of the films.
In the medical and biotechnology fields, there are many uses for peel pouches. When an operator opens the pouch by peeling the layers apart, the contents of the pouch can be withdrawn from the pouch without violating sterility. In contrast, a simple bag must be cut open with a scissors or knife which can carry contamination into the interior of the bag or damage the contents.
The peeling process is typically facilitated by the use of a chevron shaped seal at one end. The seal is formed by heat sealing in the shape of a "v" whose apex points outward from the interior of the pouch. Peel tabs formed from excess lengths of the back and the front films extend outward from the pouch past the chevron seal. When the operator pulls on the peel tabs, the tearing of the adhesive is started by the apex of the chevron. In addition to the use of a chevron and peel tabs, the adhesive seal must have a peel strength below the tensile strength of the films which comprise the front and back of the pouch, so that during peeling the adhesive fails but the films do not tear.
Many peel pouches consist of a clear film on one side and a paper layer on the other side. This allows visual examination of the contents while they are still in the pouch, while the paper allows penetration of gas or steam for the sterilization of the contents. The clear film is usually a dual layer film such as polyester bonded to polypropylene. The polypropylene serves as a hot melt adhesive which allows the film to be bonded to the paper backing. Upon peeling, the polypropylene delaminates a thin layer from the surface of the paper, allowing a controlled strength peel.
In the organ transplantation field, an outer pouch frequently consists of a metal foil-polyethylene laminate. This laminate has disadvantages in that it is not transparent and occasionally breaks during shipping at liquid nitrogen temperatures. In addition, the pouch is not peelable, and so has to be cut open with sterile scissors at the time of use. This creates the risk of contamination entering the interior of the pouch. The inner pouch is a nylon polyethylene laminate which is transparent, but is subject to embrittlement when frozen at liquid nitrogen temperatures.
Peel pouches heretofore have had the disadvantage of being produced from materials which are not stable at cryogenic temperatures and become extremely brittle at such storage temperatures. These pouches are therefore unsuitable for storing materials, such as biological tissue and cells, at cryogenic temperatures. Examples of this type of peel pouch are found in U.S. Pat. Nos. 4,358,015, 4,352,429, 4,190,154 and 4,121,714.
Packages are also known which can be used to store blood or other materials at cryogenic temperatures. These packages do not include peelable seals, thereby requiring the package to be cut by scissors or a knife, and risking contamination of the contents. An example of this type of package is disclosed in U.S. Pat. No. 3,942,529.